Many industrial installations in developing countries start-up as small factories, without regard for the need of compensation of reactive power, leading to significant financial losses in the long term. By improving the power factor, the customer can reduce its power demand and potentially increase efficiency of their equipment. A PIC microcontroller is used to switch capacitor banks to compensate for the reactive power. In order to determine the size of the capacitor bank needed, the microcontroller calculates the phase difference between the voltage and the current. The results obtained based on the lagging power factor for three test loads show an improvement in the power factor from 0.52 to 0.96 under different test load conditions.
The conventional methods used to switch-in capacitors to compensate for reactive power can generate large transients that are detrimental to other sensitive electrical loads in the network. Coupled with the inability of these conventional systems to accurately determine and respond to load changes, make the conventional approaches inadequate for applications involving reactive power fluctuations. As small industries continue to increase on their number of inductive motor drives in their quest to increase productivity, reactive power management becomes increasingly more important, due to economic concerns. Electricity utility companies are increasingly establishing pricing policies such that customers with poor power factors will have to pay for their reactive power as well as their active power consumption. The price of electricity relating to transmission charges has to cover apparent power (in kVA) as well as transmission losses and not just kW. The disadvantages of operating the factory at lagging power factors are readily apparent. These include penalties paid to the electricity supplier, increase in the power that must be supplied by the generators, heat losses in the conductor leading to voltage drops oversizing of conductors and cables, and the tripping of circuit breakers.
The transportation of reactive power through power lines causes significant losses, decrease in the stability of the network and voltage drops at the consumption points. In order to avoid this, reactive power compensation, reactive power compensation devices can be installed closer to the loads. A previous study [
In this paper, a microcontroller based controller continuously takes measurements of the phase voltages and line or load currents, and updates a capacitor switching sequence at each period of the waveform as required [
The frequency of the alternations between a positive maximum value VM and a negative minimum value –VM of AC signal is denoted f. The amplitude (maximum or peak value) of the voltage is denoted VM (in volts), but the quantity generally specified is the effective value denoted simply by V (or Veff or Vrms). For a sinusoidal waveform, the relationship between the amplitude (maximum value) VM and the rms value V is: V M = 2 ∗ V , which means that the amplitude of the voltage corresponding to 230 V is 325 V.
A very important notion in AC power flow is the phase shift, ϕ between the voltage and the current waveforms, called the power factor. Depending on the type of electrical device (resistive, inductive, capacitive), the phase shifts are different. For a purely resistive AC. circuit, the average power dissipated, P, is given by:
P = I V = I 2 R watts (1)
Apparent power, S, is an important quantity since AC systems, such as generators, motors, transformers and cables. The allowable output of such systems is usually limited not by mechanical stress but by temperature rise, and hence by the power losses. The losses are determined by the voltage and current and are
very closely associated with the power factor. Thus, the amount of electrical power required to supply a certain load on an industrial site is essentially determined by the volt-ampere ratings of the load rather than by the power (in watts) alone. The rating of a machine is defined as the maximum apparent power that it is designed to carry continuously without overheating.
S = P ± j Q (2)
where, P is the true power in watts and Q is the reactive power in volt-ampere-reactive (VAR). The absolute value of S can be determined by:
S = P 2 + Q 2 (3)
The reactive power, Q contributes nothing to the net energy transfer and yet it causes just as much loading of the equipment as if it did so. Reactive power is a term much used in power generation, distribution and utilization of electrical energy. Inductive reactive power, by convention, is defined as positive reactive power; capacitive reactive power, by convention, is defined as negative reactive power.
Q = V I sin ϕ (4)
And,
P = V I cos ϕ (5)
The power factor (PF) is defined as the ratio of the real power flowing to the load, to the apparent power. Real power is the capacity to do work in a particular time. Apparent power is the product of the current and voltage of the circuit. Due to energy stored in the load and returned to the source, or due to a non-linear load that distorts the wave shape of the current drawn from the source, the apparent power will be greater than the real power [
PF = realpower ( P ) reactivepower ( S ) (6)
Equation (6) takes the form of Equation (7) [
PF = cos ϕ (7)
Power factor correction (PFC) is a technique of counteracting the undesirable effects of electric loads that create a power factor (PF) that is less than 1. Power factor correction may be applied either by an electrical power transmission utility to improve the stability and efficiency of the transmission network or, correction may be installed by individual electrical customers to reduce the costs charged to them by their electricity supplier. In either case, capacitive Power Factor correction is applied to circuits which include induction motors as a source of reactive power, thereby reducing the inductive component of the current and consequently reducing the losses in the supply. When capacitors are connected at each starter and controlled by that starter, the configuration is known as Static Power Factor Correction.
In distribution systems, the voltage is normally controlled only at the entry point (substation), and then it sags down the distribution lines, mainly because of consumption of reactive power by consumers and the impedance of the distribution lines. A number of technologies are employed in the modern power systems to compensate for the flux of reactive power and thus to improve the power factor. These technologies compensate for the drop of voltage using controlled injections of reactive power at a few locations. Some of these existing technologies are briefly described below.
Large synchronous generators typically control their output voltage within the prescribed bounds by manipulating (usually injecting) reactive power. Control is realized via an excitation system that consists of exciter, controller and voltage measurement components [
When a synchronous motor operates at under excited and at over excited conditions, it is called a synchronous condenser. It can generate and absorb reactive power. When synchronous motor is over excited it draws leading current and works like a capacitor. When synchronous condenser provides leading current, it eliminates reactive component of power and improves power factor [
When the power factor is low due to inductive lagging current, static capacitors are connected in parallel with the devices, such capacitors provide leading current which neutralizes lagging currents and improves power factor. For three phase loads, capacitors are connected in star or delta. Static capacitor is invariably used in power factor improvement in industries [
static compensation technique in which a 3 phase load is connected with a 3 phase line. Static capacitors are connected in parallel to provide leading currents which neutralizes lagging currents and improves power factor.
The limitation in this case is that they have short service life ranging from 8 to 10 years, they are easily damaged if the voltages exceed the rated value, and once the capacitor is damaged, the repair is uneconomical [
There are no general rules applicable to every type of installation and, in theory, capacitors can be installed at any point, but it is necessary to evaluate the relevant practical and economic feasibility. Distributed power factor compensation is achieved by connecting a properly sized capacitor bank directly to the terminals of the load which requires reactive power. The installation is simple and inexpensive; capacitor and load can use the same protective devices against overcurrent and are connected and disconnected simultaneously [
Group power factor compensation is used locally in improving the power factor of groups of loads having similar functioning characteristics, as in
The system shown in
crossing detectors which converts it to a square wave signal. These pulses are fed to the two interrupt pins (RB0 and RB4) of the microcontroller. The microcontroller has an internal timer which calculates the time difference and converts it into phase angle and then using the cosine of the angle to obtain the power factor which is displayed on the LCD. If the power factor is low, the microcontroller activates a relay using its relay drivers by connecting the capacitor bank to the circuit which improves the power factor.
The block diagram of
In
The output from the zero crossing detectors is fed to the pins of PIC of microcontroller separately. The PIC uses its internal timers to measure the time duration
of signals in which their value is high or “1”. The timer is a register which counts from 0 to 255 and then start from 0 again. Timers are available in all PIC microcontrollers and when the timer overflows it generates interrupt. The timer can use internal clock or external clock to calculate the time, but this depends upon programmer.
The time difference between two waves can be easily measured using external interrupts. Whenever an interrupt signal is received on an input pin of the microcontroller, the timer starts counting and as soon as another external interrupt is received, the timer stops counting. One interrupt is generated with the help of the current signal and other interrupt with the help of the voltage signal. The timer value is stored for further use. This variable value is basically a time difference between two waves. For good results 20 to 30 values are used and their average is calculated [
Microcontroller then uses its abilities and program to calculate the phase angle and phase difference between two waveforms. When voltage crosses the zero we start the timer and stop the timer when current crosses the zero. In between both zero crossing we find out the value of count. This count gives the time gap.
Δ T = count1 countfor1 s (8)
For example, using
In
| Pre-scale Value | Count | Count for 1 second | Frequency |
|---|---|---|---|
| 1:2 | 128 | 6400 | 50 Hz |
| 1:4 | 64 | 3200 | 50 Hz |
| 1:8 | 32 | 1600 | 50 Hz |
| 1:16 | 16 | 800 | 50 Hz |
| 1:32 | 8 | 400 | 50 Hz |
| 1:64 | 4 | 200 | 50 Hz |
| 1:128 | 2 | 100 | 50 Hz |
| 1:256 | 1 | 50 | 50 Hz |
| Sr. no | Pre-scale Value | Count 1 | Count | Power factor |
|---|---|---|---|---|
| 1 | 1:2 | 24 | 128 | 0.382 |
| 2 | 1:4 | 12 | 64 | 0.382 |
| 3 | 1:8 | 6 | 32 | 0.382 |
| 4 | 1:16 | 3 | 16 | 0.382 |
| 5 | 1:32 | 2 | 8 | 0 |
| 6 | 1:64 | 1 | 4 | 0 |
| 7 | 1:128 | 1 | 2 | -1 |
| 8 | 1:256 | 1 | 1 | 0 |
power factor of the tested circuit at different pre-scale values is shown in
Angle = count1 count × 360 (9)
And then, the power factor is given by the cosine of the angle obtained from Equation (9).
Ease of
It can be seen from
The capacitor banks are automatically connected to or disconnected from the circuit using respective relays. Two capacitor banks were used in this case, each
of rating C/2 implying that they are connected in parallel. This is simply because the size of the capacitor bank has to be finite and the cost of design consideration. The relays with the best combination of capacitors provide an improved power factor. The loads used in this experiment were incandescent lamps (for resistive load), and fluorescent tubes with chokes (for inductive load).
The automatic control system for power factor improvement was first tested on Proteus software. The Proteus software is a proprietary software used mainly for electronic design testing. It is used to create schematics and electronic prints for manufacturing printed circuit boards. In this work, the simulation result is based on lagging power factor of the load. These results are for the three design loads (Pure R, Series R-L and Parallel R-L).
The simulation results are based design loads mentioned earlier. These can be extended to other load configurations.
1) Case 1:When resistive a load of 60watts is connected.
When a resistive load of 60 watts is connected, it is observed that both the current and voltage are in phase as shown
2) Case 2:When a series R-L load is connected.
When the series R-L load is connected, there is a phase difference between the voltage and current signals as shown in
supplied to the system, and the current and voltage waveforms are in phase. After the connection of the required value of capacitor, the V and I zero crossing detector signals are also in phase in accordance with the set reference value of the power factor (0.95).
3) Case 3:When a parallel R-L load is connected.
When the parallel R-L load is connected, there is a phase difference between the voltage and current signals as shown in
The power factor is a measure of the efficiency of an electrical device to consume
| Time | P (W) @ 230v P = V I cos ϕ | Pf before correction | Q (Var) Q = S 2 − P 2 | S (VA) S = P PF | I (A) |
|---|---|---|---|---|---|
| 0 | 84.7 | 0.79 | 65.7 | 107.2 | 0.466 |
| 1 | 85.40 | 0.80 | 64.1 | 106.8 | 0.464 |
| 2 | 78.66 | 0.76 | 67.3 | 103.5 | 0.450 |
| 3 | 77.5 | 0.72 | 74.6 | 107.6 | 0.468 |
| 4 | 67.4 | 0.62 | 85.3 | 108.7 | 0.473 |
| 5 | 65.96 | 0.60 | 87.9 | 109.9 | 0.478 |
| 6 | 77.61 | 0.74 | 70.6 | 104.9 | 0.456 |
| 7 | 73.73 | 0.70 | 75.2 | 105.3 | 0.458 |
| 8 | 77.33 | 0.72 | 74.6 | 107.4 | 0.467 |
| 9 | 83.60 | 0.78 | 67.1 | 107.2 | 0.466 |
power correctly when current flows through it. Electricity distribution companies usually charge for the apparent power (in kVA) consumed based on the measurement made with the energy meter. If the power factor of an installation is low, the current consumed will be high, resulting in a higher electricity bill [
The installation of capacitor banks minimises the penalties from the electricity distribution company and also reduces the apparent power (VA) consumption. There are different types of power factor compensation capacitor banks – the fixed compensation, where the whole battery is switched (On/Off), and the automatic compensation, also known as stepped compensation, where the capacitor bank is divided into several steps that are automatically switched on according to the reactive power to be compensated [
A reactive power compensation system has been designed. In the designed system, compensation is performed by monitoring continuously the lagging power factor due to the load and triggering the control action through a proper algorithm by switching in capacitor banks through different relays thereby improving the power factor of the load. The simulation results based on the lagging power factor for three test loads showed an improvement in the power factor from 0.52 to 0.96 under the test load conditions. The reduction in current drawn relives the system and allows more loads to be connected without overloading the system. When the reactive power compensation device is used, the total apparent power demand is reduced due to improved power factor. The use of the PIC microcontroller is to reduce the cost and increase accuracy.
In the perspectives, it will be possible to look at the elimination of harmonics from the distribution networks. The modernisation of industrial processes and the sophistication of electrical machines and equipment have led to significant development of power electronics in recent years. For electrical networks, these systems represent so-called “non-linear” loads. These non-linear loads inject non-sinusoidal currents into the network. These currents are formed by a fundamental frequency component of the network, plus a series of superimposed currents, with frequencies that are multiples of the fundamental, called harmonics. The immediate effects of these harmonics include degradation of the power factor, reduction in motor power, and overloading of cables, transformers and motors; leading to reduction in the life span of motors and transformers, and the deterioration of capacitor banks.
The authors thank the authorities of the University of Bamenda, ENSPY, University of Yaoundé I, Cameroon, and GTTI, The Gambia, for their institutional support.
The authors received no specific funding for this study.
The authors declare no conflicts of interest regarding the publication of this paper.
Mbinkar, E.N., Asoh, D.A. and Kujabi, S. (2022) Microcontroller Control of Reactive Power Compensation for Growing Industrial Loads. Energy and Power Engineering, 14, 460-476. https://doi.org/10.4236/epe.2022.149024